Many of recent vehicles have a rear cross traffic alert device for notifying a driver of the presence of another vehicle, etc. in a right/left area laterally behind the vehicle which is a dead area for a driver (notification area). The rear cross traffic alert device detects another vehicle approaching the notification area at the right or left laterally behind the vehicle by a millimeter-wave radar or the like, and when the other vehicle has entered the notification area, notifies (alerts) the driver of the presence of the other vehicle (for example, Patent Literature 1).
In addition, according to the rear cross traffic alert device disclosed in Patent Literature 1, when the vehicle having the rear cross traffic alert device is to be parked, the notification area (detection angle for notification target) laterally behind the vehicle is changed in accordance with the entry angle of the vehicle to a parking space upon the parking, whereby notification about a necessary area can be performed without notification about an unnecessary area.